11-year-old Border Terrier doesn't want to eat

[Fauve]

The vet has checked him. I am giving him Cesar Senior.

Any thoughts please?

A sore tooth? Did the vet draw a blank?

Try a different food.
Chappie or Pedigree Chum, not popular choices in this forum but I've found them reliable as a quick change of diet to stimulate a dogs appetite. Once he's eating again he'll probably go back to his normal food no bother.